Cold and snow keep battering the Midwest and East, and even Atlanta was temporarily paralyzed. California has been bone dry. Alaska set heat records.

The wild winter somehow became even more wicked Thursday morning when the national average temperature plunged to a brutal 11 degrees — the lowest temperature of a season of extremes

A weather weary nation asks a simple question: Why?

The answer is the jet stream, the river of air that dictates our weather. Normally the jet stream stays in Canada or the northern U.S., going west to east in a somewhat straight line. But this winter it has plunged south, creating high pressure ridges and low pressure troughs and taking cold polar air south and east and leaving warm, dry weather to the west.

“We are having an unusual jet stream that’s giving us crazy cold weather in the East and the ridiculously resilient ridge as it’s called in California,” said Weather Underground meteorology director Jeff Masters.

Q: Why is the jet stream doing this?

A: There are three different forces probably at work here, but scientists still need to do more research, said Derek Arndt, of the National Climatic Data Center in Asheville, N.C. One is just the random natural variability of daily weather. Another is a mid-length weather feature called the Pacific Decadal Oscillation — think of it as a cousin of El Nino — that warms the northern Pacific and helps push the jet stream south. And finally, a new and controversial theory is that a warmer Arctic region and shrinking summer sea ice from man-made global warming has shifted jet stream patterns, making it wavier and bringing more unpredictable weather.

Q: Is it unusual for the weather pattern to last this long?

A: It doesn’t happen often, but it’s not that unusual either, said Bruce Terry, of the National Weather Service’s Weather Prediction Center in College Park, Md.

Q: So how cold was Thursday?

A: The national average temperature of 11 degrees is the coldest of this winter and will likely be the coldest of the season, according to calculations by Weather Bell Analytics meteorologist Ryan Maue. It was computed from temperatures at 7 a.m. EST in the Lower 48 states.

The lowest was minus 34 in Montana and several areas were minus 20, according to the National Weather Service.

Q: Has this been a record winter?

A: No. Given the unusual heat in the West and the cold in the East, they almost balance each other, Masters and Arndt said. So when the final monthly statistics come out, January in the U.S. won’t be near record cold.

“When you compare it to the 20th century, it was still cold, but not dramatically cold,” Arndt said.

Q: Was this just a U.S. thing?

A: No. Parts of South America and Australia have had much warmer than normal weather. Parts of Europe have been cold and stormy, others record warm. For much of January, Greenland was 8 degrees warmer than normal.

Q: When will it end?

A: Soon enough. In Northern California, heavy rains are coming. A predicted eastern winter snowstorm this weekend is looking less mighty than it did a few days ago. It’s not soon enough for the meteorologists who predict it.